Time delay fuse



Dec. 11, 1956 E. v. SUNDT TIME DELAY FUSE:

Filed April 28, 1954 United States Patent TIME DELAY FUSE Edward V.Sundt, Wilmette, 1li., assignor to Smidt Engineering Company, DesPlaines, Ill., a corporation of Illinois Application April 28, 1954,Serial No. 426,077

14 Claims. (Cl. 2043-123) The principal object of this invention is toprovide an improved time delay fuse for electric circuits which providessubstantially instantaneous action upon the occurrence of highoverloads, which provides a time delayed action upon lower sustainedoverloads, which prevents needless fuse blowing on harmless overloads,which utilizes a new and improved principle of operation, which is of anew and improved construction, which is foolproof in operation, andwhich can be inexpensively manufactured.

Basically, the time delay fuse of this invention is a heat operateddevice wherein heat is generated therein through the passage of electriccurrent and absorbed over time up to a calibrated point of fusion inorder to secure the desired delay. Two physical factors are utilized tosecure the time delay, the specific heat of the entire fuse element andthe latent heat of fusion of a body of fusible metal purposely added tothe element.

Briefly, the time delay fuse of this invention includes an electricallyconductive fuse element formed from a fusible metal and having betweenits ends a receptacle portion and a resistance heating portion oflimited cross sectional area adjacent the receptacle portion. A body offusible metal of lower melting point than that of the fuse element issecured in the receptacle portion of the fuse element in intimateContact therewith so as to provide high thermal conductivitytherebetween. Electrical terminals are connected to the ends of the fuseelement for incorporating the same in an electric circuit to beprotected.

A sustained overload in the electric circuit causes the resistanceheating portion of the fuse element to heat the fuse element and thebody of fusible metal substantially as a unit. The heating of theresistance heating portion of the fuse element to the melting pointthereof for breaking the electric circuit being protected is delayed bythe specic heat and the latent heat of fusion of the body of fusiblemetal. ln this way substantial time delays in blowing of the fuse at theresistance heating portion thereof may be obtained. At the same time ahigh overload, such as caused by a short circuit, will substantiallyinstantaneously blow the fuse at the resistance heating portion thereof.

Preferably, the fuse element of the time delay fuse of this inventionincludes a pair of spaced apart receptacle portions provided with thelower melting point fusible metal in each, an inner resistance heatingportion extending therebetween and an outer resistance heating portionextending outwardly from each receptacle portion. Here, the delayedblowing of the fuse normally takes place at the inner resistance heatingportion, where blowing conditions may be more readily calibrated andcontrolled, while instantaneous blowing may take place at any one ormore of the three resistance heating portions. The fuse element may beencased in a tubular closure of electrical insulating material providedwith metallic end 2,773,961 Patented Dec. 11, 1956 ICC caps to which theends of the fuse element are electrically connected to form a cartridgetype of fuse. Of course, other types of closures may be utilized.

The fuse element may be formed from a sheet of fusible metal and thereceptacle portions may be substantially tubular in shape formed bysuitably stamping and bending the sheet. The bodies of fusible metal maybe secured in the tubular receptacle portions merely by dipping the fuseelement in a molten pool of the lower melting point fusible metal.Substantially any kind of fusible metals may be used for the fuseelement and the fusible bodies depending upon the fusing conditions tobe met. They may be basic metals or alloys of metals, eutectic alloysbeing preferred for the lower melting point fusible bodies since theirmelting conditions may be more readily determined and calibrated.Preferably, the outer resistance heating portions of the fuse elementare bent substantially normal to the axes of the cylindrical receptacleportions to compensate for thermal expansion and contraction.

Other objects of this invention reside in the details of construction ofthe time delay fuse and in the cooperative relationships between thecomponent parts thereof.

Other objects and advantages of this invention Will become apparent tothose skilled in the art upon reference to the accompanying specication,claims and drawing in which:

Fig. l is a perspective view of a time delay fuse constructed inaccordance with this invention;

Fig. 2 is a sectional view taken substantially along the line 2-2 ofFig. l;

Fig. 3 is one form of a blank formed from a sheet of fusible metal fromwhich the fuse element is fabricated;

Fig. 4 is another view similar to Fig. 3 showing another form of blankfor fabricating the fuse element;

Fig. 5 is a perspective view of a fuse element formed into shape andincluding the bodies of fusible metal;

Fig. 6 is a sectional view taken substantially along the line 66 of Fig.2;

Fig. 7 is a sectional view taken substantially along the line 7 7 ofFig. 2.

The time delay fuse of this invention is generally des ignated at 10. Itincludes a fuse element 11 formed from a sheet of fusible metal. Theblank from which the fuse element is fabricated is illustrated in Fig.3. It includes a pair of parts 12 and 13 which ultimately form thesubstantially tubular cavities. Extending between the parts 12 and 13 isa resistance heating portion 14 of limited cross sectional area. Asillustrated the part 14 is provided with a cut-out 1S to limit the crosssectional area. When current is passed through the fuse element 11 theportion 14 of limited cross sectional area is heated thereby. Extendingfrom the outer end of each part 12 and 13 is a tab 16 and a tab 17,respectively. These tabs 16 and 17 are provided with cut-outs 18 and 19for providing the tabs with a limited cross sectional area so that thetabs 16 and 17 also form resistance heating portions of the fuse element11. As shown in Fig. 4 the tabs 16 and 17 may be made narrower toprovide resistance heating portions of limited cross sectional area.

ln fabricating the ,fuse element the par-ts 12 and 13 are formed intosubstantially cylindrical cavities as illustrated in Fig. 5. The tabs 16and 17 adjacent the outer ends of the `receptacle portions 12 and 13 arebent substantially normal to the axes of the receptacle portions `asshown in Fig. 5. After the fuse element has been so formed orfabricated, it is then first dipped in a flux and immediately thereafterinto a molten pool of `fusible metal of lower melting point. ln sodoing, the fusible metal flows into the tubular receptacles 12 and 13and is maintained in there by capillary attraction. When the fuseelement is removed from the molten pool, the fusible metal solidifies in.the receptacle portions and is thereby pemanently sccured therein inintimate contact therewith so as `to provide high thermal conductivitytherebetween. The fuse element is then provided with `a rinse to removeexcess tiux and it is then ready for incorporation into a suitableclosure.

rllhe closure for the fuse element may include a tubular closure 25formed of electrical insulating material such as glass or the like. Theends of the tubular closure 25 are preferably turned inwardly slightlyas indicated yat 26 and 27. The fuse element is inserted in the closure25 and metallic end caps 28 and 29 are placed over the ends of theclosure 25. The end caps 2S `and 219 are secured in place on the tubularclosure 25 and are electrically connected to the ends of the fuseelement il by means of solder indicated at 3@ and 3l. The end caps 2&5and 29 therefore provide electrical connections for connecting the fuseof this invention into an electrical circuit to be protected.

When a sustained overload occurs in the electric circuit, it causes theresistance heating portions i4, )i6 and il/ of limited cross sectionalarea to heat the fuse element l?. and the bodies of fusible metal 27.and 23 substantially as a unit. The heating of the resistance heatingportions of the fuse element to the melting point thereof for breakingthe electric circuit being protected is delayed by the specific heat ofthe unit as a whole and the latent heat of fusion of the bodies of thefusible metal. in this way substantial time delays in blowing of thefuse at .the resistance heating portions thereof may be obtained. Sincethe resistance heating portion M is located between the bodies offusible metal 22 and Z3 and will probably be of higher temperature thanthe other resistance heating portions lo `and l?, 4the fuse will blow onsustained overload conditions at this point rather than the other twopoints. At the same time a high overload condition such as caused by ashort circuit will substantially instantaneously blow the fuse at anyone or more of the resistance heating portions i4, 16 and i7. Becausethe resistance heating portions i6 .and 17 are bent substantially normalto the .axis .of the cylindrical receptacle portions l2 and i3, thesebent portions may Hex upon expansion and contraction of the fuse elementdue to temperature conditions and therefore the fuse of this inventionis fully compensated for thermal expansion and contraction and fatiguingof the fuse element is maintained at a minimum.

Substantially any kind of fusible metals may be used, depending,y uponthe fusing conditions to be met. For example, the fuse element il may beformed from sheet zinc or the like. Different ampere ratings of thefiusc element may be obtained by varying the thickness of the zinc sheetand in practical applica-tion this ranges from .OOlS to .015 inch forsmall fuses. Accordingly this design provides an inexpensive means forsecuring a great number of ampere, or intermediate ampere, ratings. Inregard to the melting points of the fusible metal bodies 22 and 23 it is.preferable not to have the melting point much lower than 288 F., whichis the eutectic of tin and bismuth, nor is it desirable to have .themaximum more than 690 F., which is the eutectic of cadmium and silver.The eutectic of tin and lead, melting at approximately 360 F., is a goodcompromise temperature, and is high enough above normal ambienttemperatures so that the blo-wing characteristics of the fuses are notinfluenced greatly thereby.

In service under operating conditions, the assembled fuse of thisinvention will carry 110% of rated current for a minimum of four hours.if the current is increased to any value from about 135% of rating andup to less than 400% of rating, the resistance heating portions of thefuse element generate enough heat to bring the fusible metal in the heatreceptacles up to the melting point thereof. Up to this point, delay infuse action is due principally to the specific heat of the fuse elementand the bodiesv `of fusible metal. Above this point, the fusion metal ischanged from solid to liquid., and no further temperature rise takesplace until all cf the fusible metal has .changed state, and a furtherdelay is thus obtained due to the latent heat of fusion. Depending onthe alloy used in the bodies of fusible metal, this del-ay is severaltimes that obtained by the specific heat only. At of rating, the fuse ofthis invention will blow in about one hours time while at 400% rating,the fuse will blow in about five .scconds time. Above 400% rating, thefuse will blow at a time inversely proportional to the overload.

While for purposes of illustration one basic form of this invention hasbeen disclosed, other forms thereof may become apparent to those skilledin the art upon reference to this disclosure and there-fore thisinvention is to be limited only by the scope of the appended claims.

I claim as my invention:

l. A time delay fuse comprising an electrically conductive single piecefuse element formed from a fusible :letal having a predeterminedrelatively high melting point and having between its ends a receptacleportion and a resistance heating portion of limited cross sectional areaat each end of and adjacent to the receptacle portion, a body of fusiblemetal having a predetermined relatively low melting point and secured inthe receptacle portion of the fuse element in intimate contact therewithso ,as to provide a high thermal conductivity therebetween, `andelectrical terminals connected to the ends of the fuse element forincorporating the same in `an electric circuit to be protected, asustained Voverload in the electric circuit causing the resistanceheating portions of the fuse element to heat the fuse element and thebody of fusible metal substantially as a unit, and the heating of theresistance heating portions of the fuse element to the relatively highmelting point thereof for breaking the electric circuit `being delayedby the specific heat .and the latent heat of fusion of the body of `therelatively low inciting point fusible metal.

2. A time delay fuse comprising `an electrically conductive elongatedsingle piece fuse element formed from a fusible metal having apredetermined relatively high melting point and having between its endsa pair of spaced apart receptacle portions, a resistance heating portionof limited cross sectional area adjacent to and between the receptacleportions and a resistance heating portion at and adjacent to the outerend of each receptacle portion, a body of fusible metal having apredetermined relatively low melting point and secured in eachreceptacle portion of the fuse element in intimate contact therewith s0as to provide high thermal conductivity therebetween, and electricalterminals connected to the ends of the fuse element for incorporatingthe same in an electric circuit to be protected, a sustained overload inthe electric circuit causing the resistance heating portions of the fuseelement to heat the fuse element and the bodies of fusible metalsubstantially as a unit, and the heating of the resistance heatingportions of the fuse element to the relatively high melting pointthereof for breaking the electric circuit being delayed by the specificheat and latent heat of fusion of the bodies of the relatively lowmelting point fusible metal.

3. A time delay fuse comprising an electrically conductive elongatedsingle piece fuse element formed from a sheet of fusible metal having apredetermined relatively high melting point and having between its endsa substantially tubular part forming a receptacle portion and a tab oflimited cross sectional arca extending from each end of the tubular partand each forming a resistance heating portion adjacent to the receptacleportion, a body of fusible metal having a predetermined relatively lowmelting point and secured `in the receptacle portion of the fuse elementin intim-ate contact therewith so as to provide high thermalconductivity therebetween, and electrical terminals connected to theends of the fuse element for incorporating the same in an electriccircuit to be protected, a sustained overload in the electric circuitcausing the resistance heating portion of the fuse element to heat thefuse element and the body of fusible metal substantially as a unit, andthe heating of the resistance heating portions of the fuse element tothe relatively high melting point thereof for breaking the electriccircuit being delayed by the specific heat and the latent heat of fusionof the body of the relatively low melting point fusible metal.

4. A time delay fuse comprising an electrically conductive elongatedsingle piece fuse element formed from a sheet of fusible metal having apredetermined relatively high melting point and having between its endsa substantially tubular part forming a receptacle portion and a tab oflimited cross sectional area extending from each end of the tubular partand each forming a resistance heating portion adjacent to the receptacleportion, a body of fusible metal having a predetermined relatively lowmelting point and secured in the receptacle portion of the fuse elementin intimate contact therewith by dipping the fuse element in a moltenpool of said lower melting point fusible metal so as to provide highthermal conductivity therebetween, and electrical terminals connected tothe ends of the fuse element for incorporating the same in an electriccircuit to be protected, a sustained overload in the electric circuitcausing the resistance heating portion of the fuse element to heat thefuse element and the body of fusible metal substantially as a unit, andthe heating of the resistance heating portions of the fuse element tothe relatively high melting point thereof for breaking the electriccircuit being delayed by the specific heat and the latent heat of fusionof the body of the relatively low melting point fusible metal.

5. A time delay fuse comprising an electrically conductive elongatedsingle piece fuse element formed from a sheet of fusible metal having -apredetermined relatively high melting point and having between its endsa pair of spaced apart substantially tubular parts forming a pair ofspaced apart receptacle portions, a part of limited cross sectional areaextending between the tubular parts and forming a resistance heatingportion adjacent to and between the receptacle portions, and a tab oflimited cross sectional area extending from the outer end of eachtubular part and each forming a resistance heating portion at andadjacent to the outer end of each receptacle portion, a body of fusiblemetal having a predetermined relatively low melting point and secured ineach receptacle portion of the fuse element in intimate contacttherewith so as to provide high thermal conductivity therebetween, andelectrical terminals connected to the ends of the fuse element forincorporating the same in an electric circuit to be protected, asustained overload in the electric circuit causing the resistanceheating portions of the fuse element to heat the fuse element and thebodies of fusible metal substantially as a unit, and the heating of theresistance heating portions of the fuse element to the relatively highmelting point thereof for breaking the electric circuit being delayed bythe specific heat and latent heat of fusion of the bodies of therelatively low melting point fusible metal.

6. A time delay fuse comprising an electrically conductive elongatedsingle piece fuse element formed from a sheet of fusible metal having apredetermined relatively high melting point and having between its endsa pair of spaced apart substantially tubular parts forming a pair of thefuse element in intimate contact therewith by dip` ping the fuse elementin a molten pool of said lower melting point fusible metal so as toprovide high thermal conductivity therebetween, and electrical terminalsconnected to the ends of the fuse element for incorporating the same inan electric circuit to be protected, a sustained overload in theelectric circuit causing the resistance heating portions of the fuseelement to heat the fuse element and the bodies of fusible metalsubstantially as a unit, and the heating of the resistance portions ofthe fuse element to the relatively high melting point thereof forbreaking the electric circuit being delayed by the specific heat andlatent heat of fusion of the bodies of the relatively low melting pointfusible metal.

7. A time delay fuse comprising an electrically conductive single piecefuse element formed from a fusible metal having a predeterminedrelatively high melting point and having between its ends asubstantially tubular receptacle portion and a resistance heatingportion of limited cross sectional area at each end of and adjacent tothe receptacle portion, a body of fusible metal having a predeterminedrelatively low melting point and secured in the receptacle portion ofthe fuse element in intimate contact therewith so as to provide highthermal conductivity therebetween, and electrical terminals connected tothe ends of the fuse element for incorporating the same in an electriccircuit to be protected, a sustained overload in the electric circuitcausing the resistance heating portion of the fuse element to heat thefuse element and the body of fusible metal substantially as a unit, andthe heating of the receptacle heating portions of the fuse element tothe relatively high melting point thereof for breaking the electriccircuit being delayed by the specific heat and the latent heat of fusionof the body of the relatively low melting point fusible metal.

8. A time delay fuse comprising an electrically conductive single piecefuse element formed from a fusible metal having a predeterminedrelatively high melting point and having between its ends asubstantially tubular receptacle portion and a resistance heatingportion of limited cross sectional area at each end of and adjacent tothe receptacle portion, a body of fusible metal having a predeterminedrelatively low melt-ing point and secured in the receptacle portion ofthe fuse element in intimate contact therewith by dipping the fuseelement in a molten pool of said lower melting point fusible metal so asto provide high thermal conductivity therebetween, and electricalterminals connected to the ends of the fuse element for incorporatingthe same in an electric circuit to be protected, a sustained overload inthe electric circuit causing the resistance heating portion of the fuseelement to heat the fuse element and the body of fusible metalsubstantially as a unit, and the heating of the receptacle heatingportions of the fuse element to the relatively high melting pointthereof for breaking the electric circuit being delayed by the specificheat and the latent heat of fusion of the body of the relatively lowmelting point fusible metal.

9. A time delay fuse comprising an electrically conductive elongatedsingle piece fuse element formed from a fusible metal having apredetermined relatively high melting point and having between its endsa pair of spaced apart substantially tubular receptacle portions, aresistance heating portion of limited cross sectional area adjacent toand between the receptacle portions and a resistance heating portion atand adjacent to the outer end of each receptacle portion, a body offusible metal having a predetermined relatively low melting point andsecured in each receptacle portion of the fuse element in intimatecontact therewith so as to provide high thermal conductivitytherebetween, and electrical terminals connected to the ends of the fuseelement for incorporating the same in an electr-ic circuit to beprotected, a sustained overload in the electric circuit causing theresistance heating portions of the fuse element to heat the fuse elementand the bodies of fusible metal substantially as a unit, and the heatingof the resistance heating portions of the fuse element to the relativelyhigh melting point thereof for breaking the electric Icircuit beingdelayed by the speciiic heat and latent hea-t of fusion of the bodies ofthe relatively low melting point fusible metal.

l0. A time delay fuse comprising an electrically conductive elongatedsingle piece fuse element formed from a fusible metal having apredetermined relatively high melting point and hav-ing between its endsa pair of spaced apart Substantially tubular receptacle portions, aresistance heating portion of limited cross sectional area adjacent toand between the receptacle portions and a resistance heating portion atand adjacent to the outer end of each receptacle portion, a body offusible metal having a predetermined relatively low melting point and inintimate contact therewith by dipping the fuse element in a molten poolof said lower melting point fusible metal so as to provide high thermalconductivity therebetween, and electrical terminals connected to theends of the fuse element for incorporating the same in an electriccircuit to be protected, a sustained overload in the electric circuitcausing the resistance heating portions of the fuse element to heat thefuse element and the bodies of fusible metal substantially as a unit,and the heating of the resistance heating portions of the fuse elementto the relatively high melting point thereof for breaking the electriccircuit being delayed by the specific heat and latent heat of fusion ofthe bodies of the relatively low melting point fusible metal.

ll. A `time delay fuse comprising an electrically conductive elongatedsingle piece fuse element formed from a fusible metal having apredetermined relatively high melting point and having between its endsa substantially tubular receptacle portion and a resistance heatingportion of limited cross sectional area at each end of and adjacent tothe receptacle portion, a body of fusible metal having a predeterminedrelatively low melting7 point and secured in the receptacle portion ofthe fuse element in intimate Contact therewith so as to provide highthermal conductivity therebetween, a tubular closure of electricalinsulating material for the fuse element, electrically conductive capssecured to the ends of the tubular closure and forming electricalterminals for connecting the fuse in an electric circuit to beprotected, means for electrically connecting the ends of the fuseelement to the caps, said resistance heating portions of the fuseelement being bent substantially normal to the axis of the tubularreceptacle portion adjacent the ends thereof to compensate for thermalexpansion and contraction, a sustained overload in the electric circuitcausing the resistance heating portions of the fuse element to heat thefuse element and the body of fusible metal substantially as a unit, andthe heating of the resistance heating portions of the fuse element tothe relatively high melting point thereof for breaking the electriccircuit being delayed by the specifi-c heat and the latent heat offusion of the body of the relatively low melting point fusible metal.

l2. A time delay fuse comprising an electrically conductive elongatedsingle piece fuse element formed from a fusible met-al having apredetermined relatively high melting point and having between its endsa pair of spaced apart substantially tubular receptacle portions, aninner resistance heating portion of limited cross sectional areaadjacent to and between the receptacle portions and an outer resistancelheating portion of limited cross sectional area at and adjacent to theouter ends of each receptacle portion, a body of fusible metal having apredetermined relatively low melting point and secured in eachreceptacle portion of the fuse element in intimate contact therewith soas to provide high thermal conductivity therebetween, a tubular closureof electrical insulating material for the fuse element, electricallyconductive caps secured to the ends of the tubular closure and formingelectrical terminals for connecting the fuse in an electric circuit tobe protected, means for electrically connecting the ends of the fuseelement to the caps, said outer resistance heating portions of the fuseelement being bent substantially normal to the axis of the tubularreceptacle portions adjacent the outer ends thereof to compensate forthermal expansion and contraction, a sustained overload in the electriccircuit causing the resistance heating portions of the fuse element toheat the fuse element and the bodies of fusible metal substantially as aunit, and the heating of the resistance heating portions of the fuseelement to the relatively high melting point thereof for breaking theelectric circuit being delayed by the specific heat and latent heat offusion of the bodies of the relatively low inciting point fusible metal.

13. A time delay fuse comprising an electrically conductive elongatedsingle piece fuse element formed from a sheet of fusible metal having apredetermined relatively high melting point and having between its endsa substantially tubular part forming a receptacle portion and a tab oflimited cross sectional area extending from each end of the tubular partand each forming a resistance heating portion adjacent to the receptacleportion, a body of fusible metal having a predetermined relatively lowmelting point and secured in the receptacle portion of the fuse elementin intimate Contact therewith so 'as to provide high thermalconductivity therebetween, a tubular closure of electrical insulatingmaterial for the fuse element, electrically conductive caps secured tothe ends of the tubular closure and forming electrical terminals forconnecting the fuse in an electric circuit to be protected, means forelectrically connecting the ends of the fuse element to the caps, saidresistance heating portions of the fuse element being bent substantiallynormal to thc axis ol' the tubular receptacle portion adjacent the endsthereof to compensate for thermal expansion and contraction, a sustainedoverload in the electric circuit causing the resistance heating portionsof the fuse element to heat the ruse element and the body of fusiblemetal substantially as a unit, and the heating of the resistance heatingportions of the fuse element to the relatively high melting pointthereof for breaking the electric circuit beng delayed by the specificheat and the latent heat of fusion of the body of the relatively loWmelting point fusible metal.

14. A time 4delay fuse `comprising an electrically conductive elongatedsingle piece fuse element formed from a sheet of fusible metal having apredetermined relatively high melting point and having between its endsa pair of spaced apart substantially tubular parts forming a pair of`spaced apart receptacle portions, a part of limited cross sectionalarea extending between the tubular parts and forming a resistanceheating portion adjacent to and between the receptacle portions, and atab of limited cross sectional area extending from the outer of eachtubular part and each forming a resistance heating portion at andadjacent to the outer end of each receptacle portion, a body of fusiblemetal having a predetermined relatively low melting point and secured ineach receptacle portion of the fuse element in intimate cont-acttherewith, a tubular closure of electrical insulating material for thefuse element, electrically conductive caps secured to the ends of thetubular closure and forming electrical terminals for connecting the fusein an electric circuit to be protected, means for electricallyconnecting the chas of the fuse element to the caps, said outer resi eating portions of the fuse element being bent substa illy normal to theaxis ot the tubular receptacle portions adjacent the outer ends thereofto compensate for thermal expansion and contraction, a sustainedoverload in the electric circuit causing the resistance heating portionsof the fuse element to heat the fuse element and the bodies of fusiblemetal substantially as a unit, and the heating of the resistance heatingportions of the fuse element to the relatively high melting pointthereof for breaking the electric circuit being delayed by the specificheat and latent heat of fusion of the bodies of the relatively lowmelting point fusible metal.

(References on following page) se s References Cited in the le of thispatent UNITED STATES PATENTS Reynolds et al Oct. 13, 1908 LippincottJuly 8, 1924 Bird Feb. 28, 1928 10 Steinmayer Nov. 7, 1933 Jung et al.Sept. 29, 1936 Baenziger June 19, 1951 Laing July 7, 1953 Baenziger May11, 1954

